A papule is a small, solid, raised elevation of the skin, typically less than one centimeter in diameter, with no visible fluid content. The elevation is produced by localized cellular proliferation, edema within the dermis, or infiltration of inflammatory cells. It is one of the primary lesion types used to classify skin abnormalities in equine dermatology, distinguished from a vesicle by the absence of a fluid-filled cavity and from a nodule by its smaller diameter and more superficial origin.
In horses, papules occur in several contexts. Urticaria (hives) produces transient, well-demarcated papules or wheals scattered across the neck, flanks, and trunk, typically in response to insect bites, contact allergens, or feed antigens. Folliculitis, a bacterial or fungal infection of hair follicles, yields papules and pustules at the follicle openings, most commonly along the back and girth area under tack. Dermatophytosis (ringworm) begins as small papules that progress to crusted, alopecic patches. Rain rot (dermatophilosis) also presents with papular eruptions that mat the hair into tufted crusts under wet conditions.
Clinical evaluation describes a papule by its size, distribution, color, firmness, and any progression toward a pustule or vesicle. The evolution of a papule over hours to days is diagnostically useful: rapid resolution with no crust suggests urticaria; slow maturation to a pustule points toward folliculitis or dermatophytosis. For causes, diagnosis, and treatment of papular conditions, see the equine health authority. See also Lesion for the broader classification of primary skin changes in horses.